Lo　art Italian Musical Terms-"Pur Di Cesti" And "Tu Lo Sai"?

Italian Musical terms-"Pur di cesti" and "Tu lo sai"? - lo　art

What do the terms "Pur di Cesti" and "Tu lo sai" means? They are used in the middle of the conditions of the vocal art class. I think the "Pur di Cesti" may "only"?